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<br />OD2794 <br /> <br />process whereby saturated, loose, cohesion less sediments (usually sands and <br />silts) are transformed from a solid state into a liquefied state. Failure is <br />usually triggered by rapid ground motion such as that experienced during an <br />earthquake, but can also be artificially induced. <br /> <br />When coherent material, either bedrock or soil, rests on materials that <br />liquefy, the upper units may undergo fracturing and extension and may then <br />subside, translate, rotate, disintegrate, or liquefy and flow (Varnes, 1978). <br /> <br />Lateral spreading in fine-grained materials on shallow slopes is usually <br />progressive. The failure starts suddenly in a small area and spreads <br />rapidly. Often the initial failure is a slump, but in some materials movement <br />often occurs for no apparent external reason (Varnes, 1978). <br /> <br />Flows <br /> <br />Creep: Creep is the imperceptibly slow, steady, downward movement of <br />slope-forming soil or rock. Movement is caused by shear stress sufficient to <br />produce permanent deformation, but too small to produce shear failure (AGI). <br />Hansen (1984) distinguishes three types of creep: 1) seasonal; movement <br />within the depth of soil affected by seasonal changes in soil moisture and <br />soil temperature, 2) continuous; where shear stress exceeds the strength of <br />the material, and 3) progressive; where slopes are reaching the point of <br />failure by other mass movements. <br /> <br />Creep is indicated by curved tree trunks, bent fences or retaining walls, <br />tilted poles or fences, and small soil rolls or ridges (figure ). <br /> <br />Debris flow: A debris flow is a form of rapid mass movement in which a body <br />of granular solids combines with entrained air and water to form a slurry that <br />then flows downslope. Generally speaking, five conditions must be met for <br />debris flows to occur: 1) steep slopes, 2) loose rock and soil materials, 3) <br />clay minerals, 4) saturated soil conditions, and 5) rainfall or snowmelt <br />generated runoff of sufficient intensity and duration to initiate slope <br />movement. <br /> <br />Debris flow areas are associated with steep gullies and can usually be <br />identified by the presence of a debris fan at the terminus (figure __). <br /> <br />- 8 - <br />